Ion selective electrode and method of preparation thereof

ABSTRACT

An integral ion selective electrode for analysis of a potassium ion, etc., comprising: 
     (I) a support; 
     (II) an electroconductive metal layer such as a silver metal layer; 
     (III) a layer of a water-insoluble salt of said metal such as a silver chloride layer; 
     (IV) an electrolyte layer which comprises electrolyte salt of a sodium ion with the same anion as the anion of the water-insoluble salt, said electrolyte salt having a mean crystal size of not more than 8 μm, and said electrolyte layer being substantially free from a binder; and 
     (V) an ion selective layer selected from the group consisting of a potassium ion selective layer, a halogen ion selective layer and a carbonate ion selective layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ion selective electrode andprocesses for the preparation of the same. More particularly, theinvention relates to an ion selective electrode appropriately employablefor the potentiometric determination of a potassium ion, a halogen ionor a carbonate ion, and processes for the preparation of the electrode.

2. Description of Prior Arts

The ion selective electrode is a device for the potentiometricdetermination of ion concentration in an aqueous sample, and a bodyfluid such as blood or serum. Its elemental structure is disclosed, forinstance, in Japanese Patent Provisional Publication No. 52(1977)-142584and U.S. Pat. No. 4,053,381. In more detail, the ion selective electrodehas an integral structure comprising a support, an electroconductivemetal layer (e.g., deposited silver metal layer), a layer of awater-insoluble salt of said metal (e.g., a layer of silver chloride),an electrolyte layer which comprises an electrolyte salt of a cation(e.g., potassium ion or sodium ion) with the same anion as the anion ofthe water-insoluble salt (for instance, potassium chloride or sodiumchloride), and an ion selective layer.

In practically determining ion concentration by means of an ionselective electrode (i.e., half cell) having the above-describedelemental structure, the following procedures are adopted: A couple ofion selective electrodes A & B are connected via a water-retainablebridge. On the ion selective electrodes A & B are spotted a standardliquid (reference liquid) and a liquid sample, respectively, and thepotential difference between both electroconductive layers of the ionselective electrodes A & B are measured after lapse of a certain periodof time. The measured potential difference is then compared against acalibration curve to determine the concentration of the electrolyte.Alternatively, a couple of ion selective electrodes insulated from eachother by a scratched groove disclosed in Japanese Patent ProvisionalPublication No. 58(1983)-156848 can be used for the measurement in thesame manner.

The ion selective electrode is composed basically of the above-statedsimple structure and can be manufactured in the form of a small sizedchip. Accordingly, the ion selective electrode is very advantageouslyemployed for the determination of an electrolyte in a small amount of aliquid sample such as a body liquid. In most cases, a body liquid isavailable for the determination in a very limited amount. Moreover, theion selective electrode is employable as a disposable device, because itcan be formed in a simple structure and in a small size.

It has been noted, however, that a measured value obtained in the use ofa small sized ion selective electrode sometimes is not reliable. Thisarises from fluctuation of electric potential (i.e., potential drift)which often takes place in the measurement procedure. It is thought thatthe potential drift is reduced by the use of an ion selective electrodein which the thickness of each of the functional layers is increased.However, the increase of the thickness of layers not only results inincrease of cost for manufacturing the device, but also results indecrease of the sensitivity.

An improvement for obviating the occurrence of the potential drift orother disadvantageous problems has been proposed in Japanese PatentProvisional Publication No. 57(1982)-17852. This improvement comprisespreparing a binderless electrolyte layer by vapor deposition of theelectrolyte or by a series of procedures of coating an aqueouselectrolyte solution containing no binder and then drying the coatedlayer. This art provides an ion selective electrode which is reduced inoccurrence of the potential drift taking place in the conventional ionselective electrode. Nevertheless, more reduction of the possiblepotential drift is desired for the purpose of enhancing the accuracy ofthe meaurement in the use of an ion selective electrode.

It has been noted that the electrolyte layer prepared from an aqueouselectrolyte solution containing no binder by a coating-drying procedurecomprises relatively large sized crystalline electrolytes. Therefore,the crystalline elecrolytes are not distributed uniformly in the layerand the resulting electrolyte layer is apt to have a large thickness.The non-uniform electrolyte layer and thick electrolyte layer both serveto decrease the accuracy of measurement.

The preparation of an electrolyte layer by vapor deposition of anelectrolyte is disadvantageous in an industrially available scale,because the vapor deposition of an electrolyte giving a low vaporpressure is difficultly employed, and otherwise the vapor deposition ofan electrolyte decomposable at a vaporized temperature requires specificcarefulness so that no high vaporization efficiency is expected.

SUMMARY OF THE INVENTION

The present inventors have proposed an improved process for thepreparation of the electrode in Japanese Patent Application No.59(1984)-93774 (the content of which is disclosed in the U.S.application Ser. No. 732,368 and EP No. 85 105 706.7). The above processwhich comprises forming the electrolyte layer by coating an aqueoussolution containing the electrolyte salt but containing substantially nobinder on the layer of a water-insoluble salt and drying the thus coatedlayer by bringing it in contact with a stream of gas maintained at atemperature of not lower than 40° C. can obviate the occurrence of thepotential drift. The process can produce a potassium salt of a smallercrystalline size to improve the stability of the ion selective electrodefor the analysis of a potassium ion. However the above potassium saltstill tends to have larger crystalline size than that of a sodium saltwhich is produced by the same manner. Accordingly a further improvementis desired.

The present inventors have studied the ion selective electrode accordingto the philosophy that the electrolyte layer should be formed by theelectrolyte salt of a smaller size in order to obviate the occurrence ofthe potential drift. They have noted that a sodium salt used as theelectrolyte salt of the electrolyte layer even in an ion selectiveelectrode other than that for the analysis of a sodium ion has noadverse infuence upon the quality of the ion selective electrolyte suchas the analytical accuracy, and at the same time the smaller crystalsmade of a sodium salt can be more easily formed than that made of apotassium salt. They have also noted that it is possible to much morereduce the occurrence of the potential drift, by utilizing the abovephenomenon.

Accordingly, a primary object of the present invention is to provide anion selective electrode which is appropriately employable for theanalysis of a potassium ion through potentiometric determination and isreduced in the occurrence of phenomena of bringing about measurementerrors such as the potential drift.

Another object of the invention is to provide an ion selective electrodewhich is shortened in the response period at the measurement.

A further object of the invention is to provide an ion selectiveelectrode appropriately employable for the analysis of a potassium ion,a halogen ion (i.e. Cl⁻, Br⁻ or I⁻) or a carbonate ion (i.e. CO₃ ²⁻ orHCO₃ ⁻) in a body liquid.

A still further object of the invention is to provide a processappropriately employable for the preparation of an ion selectiveelectrode through potentiometric determination and is reduced in theoccurrence of phenomena of bringing about measurement errors such aspotential drift.

The present invention resides in an integral ion selective electrodecomprising:

(I) a support;

(II) an electroconductive metal layer;

(III) a layer of a water-insoluble salt of said metal;

(IV) an electrolyte layer which comprises electrolyte salts of a sodiumion with the same anion as the anion of the water-insoluble salt, saidelectrolyte salt having mean size of not more than 8 μm, and saidelectrolyte layer being substantially free from a binder; and

(V) an ion selective layer selected from the group consisting of apotassium ion selective layer, a halogen ion selective layer and acarbonate ion selective layer.

The above-described ion selective electrode can be prepared by a processwhich comprises forming the electrolyte layer by coating an aqueoussolution containing a sodium salt but containing substantially no binderon the layer of a water-insoluble salt and drying thus coated layer bybringing it in contact with a stream of gas maintained at a temperatureof not lower than 40° C.

Further, the above-described ion selective electrode can be prepared bya process which comprises forming the electrolyte layer by coating asolution containing a sodium salt but containing substantially no binderin a mixture of water and an organic solvent on the layer of awater-insoluble salt and drying the thus coated layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an ion selective electrode equipped with a bridgewhich is employed for the measurement according to the differentialmethod.

FIG. 2 shows graphically a variation with time of elecric potentialobserved in the measurement according to the direct method using an ionselective electrode of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As described hereinbefore, the elemental structure of the ion selectiveelectrode provided by the present invention which comprises a support,an electroconductive metal layer, a layer of a water-insoluble salt ofsaid metal, an electrolyte layer comprising an electrolyte salt of aalkaline metal cation with the same anion as the anion of thewater-insoluble salt, said electrolyte layer being substantially freefrom a binder, and an ion selective layer is as such known. Forinstance, such embodiment is disclosed in Japanese Patent ProvisionalPublication No. 57(1982)-17852. The ion selective electrode of theinvention can be prepared in the same manner as in the conventional artsexcept that the electrolyte layer is prepared in a different manner. Theconstitution of the ion selective electrode and materials employed forthe preparation can be determined on the basis of the disclosures inJapanese Patent Provisional Publications No. 52(1977)-142584, No.57(1982)-17852 and No. 58(1983)-211648.

For instance, the support can be prepared from a film or sheet ofplastic material such as polyethylene terephthalate. A representativeelectroconductive metal layer is a silver metal layer formed on asurface of the support by vapor deposition. In the case that theelectroconductive metal layer is a silver metal layer, the layer of awater-insoluble salt can be produced by chemical oxidation-chlorinationof the surface portion of the silver metal layer to form a silverchloride layer, or by coating a dispersion containing silver chlorideand a binder on the surface of the silver metal layer and drying thecoated layer.

On the layer of a water-insoluble salt is formed an electrolyte layer.The constitution of the electrolyte layer is a characteristic feature ofthe invention and shall be described hereinafter in detail.

The ion selective layer is capable of selecting a specific ion, andshows a high electric resistance and is a substantially electricinsulative in a dry state prior to contact with a liquid sample or areference liquid. The capability of selecting a specific ion includesnot only a property of selectively allowing permeation of the specificion or selectively responding to the specific ion but also a property ofselecting a specific ion from other ions or substances with a timedifferential enough for detecting the specific ion. Moreover, certainmaterials employable for the formation of the ion selective layer arecapable of detecting a potential difference on the ion activity changeoccurring in the course of ion exchange, thereby showing the sameproperty as the property of selecting a specific ion. This property isalso included in the capacity of selecting a specific ion.

The ion selective layer ought to be water-insoluble, because a liquidsample and a reference sample both are aqueous liquids. The ionselective layer can be either hydrophilic or hydrophobic, as far as thelayer is water-insoluble. However, a hydrophobic ion selective layer ispreferred.

The ion selective layer can be prepared in the known manner. Forinstance, a solution of an ion carrier and a hydrophobic organic binderin an ion-carrier solvent is coated on the electrolyte layer and driedto give an ion selective layer. The ion carrier is generally coated inthe amount of 0.05-10 g/m², and the thickness of the ion selective layerranges from approx. 3 μm to approx. 125 μm, preferably from approx. 5 μmto approx. 50 μm.

Examples of the potassium ion carrier include valinomycin, cyclicpolyethers, tetralactones, macrolide actins, enniatin, potassiumtetraphenyl borate and their derivatives. Examples of the ion carriersolvent include phthalates, sebacates, aromatic or aliphatic ethers andadipates. Concrete examples of the solvent are described in JapanesePatent Publication No. 58(1983)-4981. For instance, there can bementioned bromophenyl phenyl ether, 3-methoxyphenyl phenyl ether,4-methoxyphenyl phenyl ether, dimethyl phthalate, dibutyl phthalate,didodecyl phthalate, dioctylphenyl phosphate, dicresylphosphate-bis(2-ethylhexyl) phthalate, octyldiphenyl phosphate, tritolylphosphate, dioctyl adipate and dibutyl sebacate. Moreover, a greatnumber of utilizable solvents are known.

The hydrophobic organic binder can be a film-forming natural polymer,its derivative, or a synthetic polymer. Examples of the hydrophobicorganic binder include cellulose ester, polyvinyl chloride, vinylchloride-vinyl acetate copolymer, polyvinylidene chloride,polyacrylonitrile, polyurethane, and polycarbonate of bisphenol A.

Details on the ion carrier, ion carrier solvent, hydrophobic organicbinder and ion selective layer are given not only in the aforementionedJapanese Patent Publication No. 58(1983)-4981, but also in JapanesePatent Provisional Publication No. 58(1983)-156848, U.S. Pat. Nos.4,053,381, 4,171,246, and 4,214,968, and Research Disclosure No. 16113(September 1977).

Moreover, an ion exchange material can be used for the formation of theion selective layer. In this case, a response on the potentialdifference originating from change of ion activity caused by the ionexchange is measured. Appropriate ion exchange materials and preparationof the ion selective layer using these materials are described inJapanese Patent Publication No. 52(1977)-47717. A representative exampleof the ion exchange material is trialkylammonium chloride for ionexchange of a chloride ion.

The ion selective electrode of the present invention is characterized inthat the electrolyte layer comprising a sodium salt is substantiallyfree from a binder and comprises crystalline electrolytes having meansize of not more than 8 μm, preferably 0.1 to 7 μm, more preferably 0.2to 7 μm, in which the crystalline electrolyte is preferably distributeddensely and uniformly on the layer of a water-insoluble salt of a metalconstituting the electroconductive layer (hereinafter referred to as"water-insoluble" salt layer). It is also characterized in that in spiteof the electrolyte layer conprising a sodium salt, the ion selectiveelectrode is appropriately employable for the analysis of an ion otherthan the sodium ion, such as a potassium ion, a halogen ion or acarbonate ion.

Preferably, the crystalline electrolytes (electrolyte crystals) are notarranged in piles in the direction vertical to the plane of the ionselective electrode. Accordingly, the mean thickness of the electrolytelayer preferably is almost equal to the mean size of the crystallineelectrolytes.

In the ion selective electrode of the present invention, the electrolytelayer is formed by arranging uniformly sodium crystalline electrolytesof a small size without using a binder over the surface of thewater-insoluble salt layer. Then the crystalline electrolytes aredensely arranged, and a relatively thin electrolyte layer can be formed.An ion selective electrode having such electrolyte layer shows quickresponse and is prominently reduced in occurrence of the potentialdrift. Moreover, the ion selective electrode of the invention is almostfree from separation of the functional layers at the electrolyte layerwhich is sometimes observed in the use of an ion selective electrodehaving a binderless electrolyte layer. The reason is thought to residenot only in high density of the electrolyte layer and decrease of thethickness but also in that the small sized electrolyte crystals definedin the invention are at least partly engaged physically with thewater-insoluble salt layer. Particularly, where the water-insoluble saltlayer is a porous silver chloride layer, the engagement between thecrystalline electrolyte and the water-insoluble salt layer (i.e., silverchloride layer) is prominently observed.

In the case that the electrolyte layer is composed of distributedsmall-sized crystalline particles, the electrolyte layer issubstantially free from poor contact or adhesion between the electrolytelayer and the ion selective layer, which are sometimes observed in thebinder-containing electrolyte layer of the conventional ion selectiveelectrode.

According to the study of the present inventors, the sodium electrolytesalt crystals having the small mean size defined in the invention arenot formed by the conventional method which comprises procedures ofsimply coating an aqueous binderless electrolyte solution on the surfaceof the water-insoluble salt layer and drying under ambient conditions.

It has been discovered that the conventional process comprisingprocedures of coating an aqueous solution containing sodium electrolytesalt such as sodium chloride or sodium bromide on the water-insolublesalt layer and allowing the coated layer to dry at room temperaturegives crystalline electrolyte of large mean size such as not less than10 μm. The electrolyte layer composed of such large sized crystallineelectrolyte is poor in the evenness of the distribution of theelectrolytes and the thickness of the electrolyte layer. Accordingly,the potential drift is apt to appear more easily and the response timebecomes longer because of thus formed thick electrolyte layers.

The electrolyte layer of the present invention can be prepared by aprocess which comprises forming the electrolyte layer by coating anaqueous solution containing a sodium salt on the water-insoluble saltlayer and drying the thus coated layer by bringing it in contact with astream of gas maintained at a temperature of not lower than 40° C.(preferably 80°-200° C.), thereby producing a layer of of crystallineelectrolyte having mean size of not more than 8 μm (preferably 0.1-7 μm)over the water-insoluble salt layer.

Preferably, the electrolyte layer of the invention is prepared by aprocess which comprises forming the electrolyte layer by coating asolution containing a sodium salt in a mixture of water and an organicsolvent on the water-insoluble salt layer and drying the thus coatedlayer. There is not specific limitation on the drying conditions, butthe coated layer is preferably dried by a stream of gas at a temperatureof not lower than 40° C. More preferably, a temperature of a stream ofgas ranges from 50° to 200° C.

The latter process using a mixture of water and an organic solvent isparticularly preferred to produce a crystalline electrolyte of a smallersize. Moreover, this process is advantageous in the preparation of theion selective electrode in a continuous form. In more detail, the ionselective electrode is manufactured industrially by initially producingan ion selective electrode in a continuous form on a continuous plasticsheet and then dividing it to give a multiple of ion selectiveelectrodes. The use of a mixture of water and an organic solvent (whichis miscible with water) is effective not only to shorten the periodrequired for the formation of the electrolyte layer but also tofacilitate the preparation of the electrolyte layer comprisingcrystalline electrolytes densely distributed over the water-insolublesalt layer. The mixture of water and an organic solvent is preferably inthe range of 2:8 to 8:2.

Examples of the organic solvent include lower aliphatic alcohols such asmethanol, ethanol, propanol and isopropanol, aliphatic ketones such asacetone, methyl ethyl ketone and diethyl ketone, ethers such as diethylether and tetrahydrofuran, and esters of lower aliphatic acids withlower aliphatic alcohols such as ethyl acetate and butyl acetate. Theorganic solvent can be employed singly and in combination.

There is no specific limitation on the concentration of the aqueouselectrolyte solution (which may contain an organic solvent) employed forthe preparation of the electrolyte layer. Generally, the concentrationof the electrolyte in the solution ranges from approx. 0.5 to approx.20% by weight, preferably approx. 0.5 to approx. 15% by weight, morepreferably approx. 0.5 to approx. 10% by weight. An electrolyte solutionof a higher concentration within the above-defined range is preferablyemployed for enhancing the efficiency in the industrial manufacturing ofthe electrode.

The anion which is a counter ion to the the sodium ion, both consistingin the electrolyte salt of the electrolyte layer of the invention oughtto be identical to the anion of the water-insoluble salt. Accordingly,the anion is selected in consideration of the whole constitutionalconditions of the ion selective electrode. Generally, theelectroconductive layer is made of a silver metal, and thewater-insoluble salt is composed of silver chloride. For this reason,sodium chloride is generally employed as the electrolyte. Nevertheless,if the anion of the water-insoluble salt is an anion other than thechloride ion, such as a bromide ion, iodide ion, sulfonium ion orcarboxylic ion, the anion of the electrolyte salt is selected to beconsistent with the above-selected anion.

The present invention is further described by the following examples.

EXAMPLE 1

On a polyethylene terephthalate film (thickness: 188 μm, 30 mm×100 mm)was formed a silver metal layer of approx. 800 nm thick by vapordeposition under vacuum. The deposited silver metal layer is covered atboth sides by means of a liquid resist of the polymer compositiondescribed in Japanese Patent Provisional Publication No.58(1983)-102146. The center portion of the deposited silver metal layerwas removed by cutting with a bit to form an insulating area of a Ushape.

The exposed portion of the deposited silver metal layer was processed ina processing solution containing hydrochloric acid and potassiumdichromate (aqueous solution containing hydrochloric acid 36 mmol./l andpotassium dichromate 16 mmol./l) for approx. 60 sec. After theprocessing was complete, the composite element was washed with water anddried to give a filmy silver-silver chloride electrode (i.e., acomposite of the support, electrocondutive silver metal layer, and asilver chloride layer).

On the silver-silver chloride electrode film was coated an aqueousorganic solution (acetone 2.5 g.+ethanol 20 g.+water 20 g.) containing2.975 g. of sodium chloride. Then the coated layer was dried by applyingthereto an air stream (1.5 m/sec.) of 80° C. for 3 min. The weight ofthe electrolyte layer was approx. 2.2 g/m² upon dryness. Microscopicobservation indicated that the electrolyte layer consists of a greatnumber of sodium chloride crystals of diameter of 1-2 μm denselydistributed over the silver chloride layer.

On the thus formed electrolyte layer was then coated a potassium ionselective layer (thickness 25 μm) of the below-described compositionformed in the conventional manner.

    ______________________________________                                        Composition of potassium ion selective layer                                  ______________________________________                                        VYNS*                     0.9   g.                                            Dioctyl adipate           1.2   g.                                            Valinomycin               44    mg.                                           Potassium tetrakis-p-chlorophenyl borate                                                                18    mg.                                           Methyl ethyl ketone       5     g.                                            1% SH-510 (polysiloxane, methyl ethyl                                                                   50    mg.                                           ketone solution)                                                              ______________________________________                                         (Remark)                                                                      VYNS: vinyl chloridevinyl acetate copolymer available from Union Carbide      Corp.                                                                    

Thus, the ion selective electrode for the analysis of a potassium ionwas prepared.

EVALUATION OF ION SELECTIVE ELECTRODE

A liquid receiver of a plastic film provided with a couple of liquidreceiving openings was fixed onto the surface of the ion selectiveelectrode under adhesion. The two liquid receiving openings areconnected to each other with a polyester spun thread bridge. Thus, anelectrode device for the analysis of potassium ion was prepared. Theelectrode device for the potassium ion analysis is schematicallyillustrated in FIG. 1. In FIG. 1, 11 indicates the polyethyleneterephthalate film (support), each of 12a and 12b indicates thedeposited silver metal layer (the deposited silver metal layer isdivided by the scratched groove to expose the surface of the support,thereby giving two separated areas), each of 13a and 13b indicates thesilver chloride layer, 14 indicates the sodium chloride (electrolyte)layer, 15 indicates the ion selective layer, 16 indicates the liquidreceiver, each of 17a and 17b indicates the liquid receiving opening,and 18 indicates the bridge.

In one liquid receiving opening 17a was spotted a potassiumion-containing reference solution (Calibrate 2), and in another liquidreceiving opening 17b was spotted a liquid sample (Calibrate 1, 2 or 3was employed). In the measurement according to a differential method,the within-run precision was measured in the conventional manner. Theresults are set forth in Table 1.

                  TABLE 1                                                         ______________________________________                                                    K Concentration (meq/l)                                           CV (%)        Nominal     Observed    n                                       ______________________________________                                        Calibrate 1                                                                           2.20      2.8         2.7       15                                    2       2.45      4.9         5.0       15                                    3       1.49      7.9         7.8       15                                    ______________________________________                                         Remark:                                                                       The slope value of Calibrate 1 was 61.2 mV/decade.                       

In the above-described measurements, the Electrode I prepared in Example1 showed no value deviating from the expected value (±50 mV) even infifty times-repeated runs.

EXAMPLE 2

A filmy silver-silver chloride electrode (reference electrode) and theIon Selective Electrode for the potassium ion analysis prepared inExample 1 were cut off along the scratched groove and connected to eachother by means of a combination of a liquid receiver and a bridge in thesame manner as illustrared in FIG. 1.

On the reference electrode was spotted a reference liquid (Calibrate 2),while on the Ion Selective Electrode I was spotted a liquid sample(Calibrate 1, 2 or 3). The variation with time of the potential wasmeasured according to the known direct method. The results areillustrated in FIG. 2.

As is clear from FIG. 2, the potential reached to a stable value withina very short time.

EXAMPLE 3

The measurement by the same method as in Example 2 was repeated on usinga liquid sample containing potassium chloride 2.8 meq/l and sodiumchloride 99 meq/l, and a reference liquid containing potassium chloride2.8 meq/l and sodium chloride 179 meq/l. The measured potentialdifference gave 1.6 mV value. The value showed that the influence fromsodium ion in the measurement for a potassium ion gave approx. 0.1-0.2meq/l. Thus it is apparent that existence of even considerable amount ofsodium ion does not form an obstacle to the practical measurement forpotassium ion.

EXAMPLE 4

The procedures in Example 1 were repeated except that the electrolytesalt solution was containing 0.5%, 1.0%, 2.0%, 3.0%, 5.0% or 10.0% ofsodium chloride to prepare ion selective electrodes for the analysis ofpotassium ion.

On each ion selective electrode was performed the evaluation accordingto the differential method in the same manner as on the Ion SelectiveElectrodes I in Example 1. Each measurement of the within-run precisionwas similar to that in Example 1.

EXAMPLE 5

The procedures in Example 1 were repeated except that the electrolytesalt solvent consists of water to form the electrolyte layer on a filmysilver-silver chloride electrode. The formed electrolyte layer wascomposed of a great number of sodium chloride fine crystals having amean size of 2-3 μm which were densely distributed over the silverchloride layer. On the thus formed electrolyte layer was then coated apotassium ion selective layer in the same manner as in Example 1 toprepare an ion selective electrode for the potassium ion analysis.

The ion selective electrode for the potassium ion analysis was used toprepare an electrode device for the potassium ion analysis in the samemanner as in Example 1. In the potassium ion measurement of the deviceaccording to a differential method, the within-run precision wasmeasured in the conventional manner. The result was similar to that inExample 1.

EXAMPLE 6

The procedures in Example 1 were repeated except that the ion selectivelayer was replaced by the carbonate ion selective layer of thebelow-described composition to prepare an ion selective electrode forthe analysis of carbonate ion.

    ______________________________________                                        Composition of carbonate ion selective layer                                  ______________________________________                                        VYNS                      9     g.                                            Octyl trifluoroacetophenone                                                                             5     g.                                            Didodecyl phthalate       9     g.                                            Trioctylpropylammonium chloride                                                                         0.5   g.                                            ______________________________________                                    

The ion selective electrode for the carbonate ion analysis was used toprepare an electrode device for the carbonate ion analysis in the samemanner as in Example 1. In the carbonate ion measurement of the deviceaccording to a differential method, the within-run precision wasmeasured in the conventional manner. The measurement showed theprefarable result that the CV value was 3-4%.

EXAMPLE 7

The procedures in Example 1 were repeated except that the ion selectivelayer was replaced by the chloride ion (Cl⁻) selective layer of thebelow-described composition to prepare an ion selective electrode forthe analysis of chloride ion.

    ______________________________________                                        Composition of chloride ion selective layer                                   ______________________________________                                        VYNS                     0.9    g.                                            Capricote                1.3    g.                                            Didodecyl phthalate      0.05   g.                                            Trioctylpropylammonium chloride                                                                        0.05   g.                                            ______________________________________                                    

The ion selective electrode for the chloride ion analysis was used toprepare an electrode device for the chloride ion analysis in the samemanner as in Example 1. In the chloride ion measurement of the deviceaccording to a differential method, the within-run precision wasmeasured in the conventional manner. The measurement showed the veryprefarable result that the CV value was 0.5-1.5%.

We claim:
 1. An integral ion selective electrode comprising in thefollowing order:(I) a support; (II) an electroconductive metal layer;(III) a layer of a water-insoluble salt of said metal; (IV) anelectrolyte layer which comprises electrolyte salts of a sodium ion withthe same anion as the anion of the water-insoluble salt, saidelectrolyte salt having a mean crystal size of not more than 8 μm, andsaid electrolyte layer being substantially free from a binder; and (V)an ion selective layer selected from the group consisting of a potassiumion selective layer, a halogen ion selective layer and a carbonate ionselective layer.
 2. The ion selective electrode as claimed in claim 1,wherein the mean crystal size of the crystalline electrolyte ranges from0.1 to 7 μm.
 3. The ion selective electrode as claimed in claim 1,wherein the mean thickness of the electolyte layer is substantiallyequal to the mean crystal size of the crylstalline electrolyte.
 4. Theion selective electrode as claimed in any one of claims 1 to 3, whereinsaid electrolyte salt is sodium chloride.
 5. The ion selective electrodeas claimed in any one of claims 1 to 3, wherein said electroconductivemetal layer is made of silver, said layer of a water-insoluble salt isof silver chloride and said electrolyte salt is sodium chloride.
 6. Aprocess for the preparation of an integral ion selective electrodecomprising in the following order (I) a support, (II) anelectroconductive metal layer, (III) a layer of a water-insoluble saltof said metal, (IV) an electrolyte layer which comprises electrolytesalts of a sodium ion with the same anion as the anion of thewater-insoluble salt, said electrolyte salt having a mean crystal sizeof not more than 8 μm, and said electrolyte layer being substantiallyfree from a binder, and (V) an ion selective layer selected from thegroup consisting of a potassium ion selective layer, a halogen ionselective layer and a carbonate ion selective layer, which ischaracterized in that the electrolyte layer is formed by coating asolution of said electrolyte salts in a mixture of water and an organicsolvent on the layer of a water-insoluble salt and drying the thuscoated layer, thereby producing a layer of crystalline electrolytehaving a mean crystal size of not more than 8 μm over the layer of awater-insoluble salt.
 7. The process for the preparation of an integralion selective electrode as claimed in claim 6, wherein said stage ofdrying the coated layer is performed by bringing it in contact with astream of gas maintained at a temperature of not lower than 40° C. 8.The process for the preparation of an integral ion selective electrodeas claimed in claim 6, wherein said mixture of water and an organicsolvent is in the range of 2:8 to 8:2.
 9. The process for thepreparation of an integral ion selective electrode as claimed in any oneof claims 6 to 8, wherein said organic solvent is selected from thegroup consisting of a lower aliphatic alcohol, an aliphatic ketone,ether, and an ester of a lower aliphatic acid with a lower aliphaticalcohol.